The properties of a toner are set through the selection of materials and amounts of those materials of the toner. The charging characteristics of a toner are also dependent on the carrier used in a developer composition, such as, the carrier coating. Toners typically comprise as least a binder resin, a colorant and one or more external surface additives. The external surface additives are generally added in small amounts. Examples of external surface additives include, for example, silica, titanium dioxide, zinc stearate, and the like. The carrier resin interacts with the surface additives.
Toners having a triboelectric charging property within the range of about −30 μC/g and about −45 μC/g may be achieved when using smaller-sized silica particles as external additives, for example, silica particles having average sizes less than 20 nm, such as, for example, R805 (˜12 nm) and/or R972 (˜16 nm) (Evonik, N.J.). However, developability at areas of low toner area coverage degrades over time. That has been attributed to the smaller-sized additives being impacted into the toner surface over time.
The problem with smaller-sized additives was considered and as an alternative, larger-sized additives, i.e., additives having a size of 40 nm or larger, such as, for example, RX50 silica, RX515H silica or SMT5103 titania (Evonik, N.J.) were used. However, although the above problem was addressed, the toners do not exhibit as high a triboelectric charging ability and also exhibit charge through. When such developers are tested at low area coverage followed by high area coverage, the developers tend to exhibit low or wrong sign toner due to charge through, i.e., the incumbent toner in the device becomes less negative or even wrong sign, i.e., positive, and the new (fresh) toner added may charge very negative. The presence of low charge and/or wrong sign toner can result in objectionable background.
Hence, there remains an issue in providing high charge and good RH sensitivity under changing environmental conditions for carrier coating resin design. For example, there remains a need to tune the charge of the carrier resin to produce higher charge for those situations where higher charge is required. To obtain both high charge and good RH sensitivity in a single design is an unfulfilled need.
Further, toners often contain silica as a surface additive. Silica is a charge driver for a toner and is RH sensitive. Thus, new carrier designs that work well with silica to improve RH sensitivity, while maintaining high charge are needed.